This Disclosure is directed to a method and apparatus for separation of an organic solvent from aqueous solutions of inorganic salts. In various processes, an aqueous solution is recovered along with an organic solvent. The aqueous solution may be substantially pure water or may have very substantial quantities of dissolved inorganic materials in it. Potentially, the liquids can be so thoroughly mixed that separation is very difficult. Surface tension characteristics plus the difference in densities and miscibility determine the mixture.
Apparatus is available for separation. One is an electrostatic coalescer manufactured by Petrolite. An electrostatic charge in such a device separates the two liquids. In general terms, the degree of separation is enhanced if the voltage level is raised. However, when the voltage level is raised, there is a risk of arcing. This risk is increased where the liquids (one or both) are relatively conductive. Electrical conductivity of pure water is substantially nil. However, typically the water has various inorganic materials dissolved in it and is therefore made much more conductive by the presence of such inorganic materials. Many salts in solution will render the solution substantially conductive. Thus, as it becomes more conductive, this imposes limitations on the electrostatic charge which can be used to coalesce the two liquids.
The present apparatus and the related method enable separation of liquids having very close densities. For instance, it is able to separate liquids having a density difference of only about 0.01 in specific gravity The liquids are coalesced and thereby separated. The coalesced aqueous liquid is pulled from the mixture and hence reduces the conductivity of the remaining solution, especially where the aqueous solution has an inorganic salt which markedly increases the conductivity. In other words, the separation changes the electrical conductivity of the mixture and enables recovery of substantially pure organic solvent. The present apparatus can be used in two or three serially arranged stages to thereby obtain very significant purification of an organic solvent. The organic solvent can then be recycled once a desired purity has been achieved. That is, it can be used again in the same process.
The present apparatus is thus a directional flow device utilizing several plastic conical partitions around a central electrode within an upstanding cylindrical housing. Alternate cones are connected to the central electrode. The remaining cones are spaced very close to the electrode to define a narrow gap. So to speak, the electrodes are interleaved so that a tortuous flow path is defined. The flow path periodically carries the mixture adjacent to the electrode to enhance coalescence progressively as the mixture of liquids is separated. The aqueous component is separated stage by stage through the apparatus and is removed to the side for flow through a separate aqueous conduit. This conduit or tube has several entrances for aqueous collection. This defines a separate flow column for the aqueous fluid. It is heavier and therefore tends to flow downwardly by gravity action to accumulate separated aqueous liquid in the bottom of the container. While recovered aqueous fluid is flowing downwardly, the newly introduced organic solvent (with aqueous impurity therein) flows upwardly becoming more pure as it travels. When the separation in the apparatus is accomplished, there is a discharge pipe out of the top of the apparatus which delivers substantially pure organic fluid. By contrast, the aqueous fluid collected in the bottom of the apparatus is removed through an aqueous fluid outlet conduit. Progressively, the aqueous fluid is coalesced and carried away from the interleaved conical members whithin the cylindrical housing. Consequently, the aqueous fluid and organic solvent separation is accomplished notwithstanding very low density differentials.
The flow rate through the device is relatively slow. This enables removal of extremely close density aqueous fluid from the organic solvent. As one example, an organic solvent polyol which is sold under the trademark VORANOL (a trademark of the Dow Chemical Company) is used in chemical processing plants which recover a high concentration of KOH in water solution. The density difference between the two liquids may be only 0.01. This process is distinctly better than the process of recovering aqueous solution of inorganics from polyols as taught in U.S. Pat. No. 3,582,491 which relies on solvent addition to change the density differential.
While the foregoing speaks generally of the background of the present apparatus and method, and provides some description thereof, the preferred embodiment is set forth below in conjunction with the drawings: